1. Field of the Invention
The present invention relates to a semiconductor integrated circuit, and more specifically to a semiconductor integrated circuit having a trimming code setting circuit.
2. Description of Related Art
In general, in case of setting a reference voltage value or a circuit current value for an analog circuit and other circuits in a semiconductor integrated circuit, a very severe value has often been required as a catalog standard or specification. In such a case, a so-called "trimming code setting circuit" has been required in order to cause the reference voltage value or the circuit current value to be within a range defined by the catalog standard or specification.
For example, one typical trimming code setting circuit, which is incorporated in a semiconductor integrated circuit and which the inventor is aware of, incudes a current setting circuit composed a P-channel MOS transistor having a source connected to a first high voltage supply line V.sub.DD (for example +5 V) and a drain connected through a constant current source to a first low voltage supply line V.sub.SS (for example, -5 V). A gate of the P-channel MOS transistor is connected to the drain of the P-channel MOS transistor itself.
The gate of the P-channel MOS transistor of the current setting circuit is connected to a plurality of trimming code setting circuits. Each of the trimming code setting circuits includes a P-channel MOS transistor having a source connected to a second high voltage supply line V.sub.DD (for example+5 V) and a drain connected through a thin film resistor to a second low voltage supply line V.sub.SS (for example, -5 V). A gate of the P-channel MOS transistor of each trimming code setting circuit is connected commonly to the gate of the P-channel MOS transistor of the current setting circuit. Therefore, the P-channel MOS transistor of each trimming code setting circuit constitutes a mirror circuit for the P-channel MOS transistor of the current setting circuit. A connection node between the thin film resistor and the drain of the P-channel MOS transistor is of each trimming code setting circuit connected to an input of and inverter, which in turn has an output for supplying a unitary code signal to a predetermined decoder. Therefore, a circuit composed of the P-channel MOS transistor and the thin film resistor of each trimming code setting circuit is connected in parallel to a circuit compossed of the P-channel MOS transistor and the thin film resistor of the other trimming code setting circuit or circuits.
Here, since the trimming code setting circuits has the same construction, explanation of function and operation will be made on only one of the trimming code setting circuits. As mentioned above, since the P-channel MOS transistor of each trimming code setting circuit is connected in the form of a mirror circuit for the P-channel MOS transistor of the current setting circuit, a predetermined current defined by the constant current source of the current setting circuit will flow through the P-channel MOS transistor of each trimming code setting circuit. Therefore, the series-connected circuit formed of the P-channel MOS transistor and the thin film resistor of each trimming code setting circuit constitutes a so-called ratio circuit.
in ordinary cases, the thin film resistor is formed to have a resistance smaller than an ON resistance of the P-channel MOS transistor when the thin film resistor has not yet been cut off. Therefore, when the thin film resistor has not yet been trimmed, namely when the thin film resistor has not been cut off, a drain potential of the P-channel MOS transistor of the trimming code setting circuit is sufficiently lower than a logical threshold voltage of the associated inverter, so that the associated inverter generates a high level signal to the associated decoder.
On the other hand, when the thin film resistor has been cut off, the drain potential of the P-channel MOS transistor of the trimming code setting circuit becomes higher than the logical threshold voltage of the associated inverter, so that the associated inverter generates a low level signal to the associated decoder.
As seen from the above, the trimming code can be set dependently upon whether the thin film resistor is cut off or not. Namely, the inverter of the trimming code setting circuit having an uncut thin film resistor outputs a low level signal, and the inverter of the trimming code setting circuit having a cutoff thin film resistor outputs a high level signal. Thus, a plurality of trimming code setting circuits cooperate to generate a plural-bit trimming code as a whole.
Incidently, the cutoff of the thin film resistor is attained by causing a current to flow through a thin film resistor to be cut off so that the thin film resistor is fused off, or by cutting the thin film resistor to be cut off by means of a laser beam. In the former case, the drain of the P-channel transistor associated to the thin film resistor is previously connected to a pad formed of for example aluminum, and in a wafer testing process using for example a tester, a voltage is applied between the low voltage supply terminal and a pad connected to the drain of the P-channel transistor associated to a thin film resistor to be cut off, so that the thin film resistor is cut off by a electric current flowing through the thin film resistor itself.
In the semiconductor integrated circuit incorporating therein the above mentioned trimming code setting circuit, once that the trimming code has been set, the trimming code has to be premanently constant in a proper operation of the semiconductor integrated circuit after the trimming code has been set. In other words, a high degree of reliability is required. in practice, however, it is some cases that the cutoff of the thin film resistor is incomplete, and in the way of the operation of the semiconductor integrated circuit, the trimming code becomes defective or faulty. This is one important cause lowering the reliability of the semiconductor integrated circuit.